(1) Summary of the Invention
The present invention relates to DNA encoding a phage resistance protein. In particular the present invention relates to plasmid pSRQ900 containing DNA which encodes Abi900 containing 183 amino acids. The plasmid when provided in dairy starter cultures imparts phage resistance.
(2) Description of Related Art
For many years, industrial Lactococcus lactis strains have been selected for their ability to rapidly produce lactic acid and to develop flavors in dairy fermentations. Bacteriophages able to inactivate these strains have been identified as the main causative agent for fermentation failures (Jarvis et al, Intervirology 32:2-9 (1991)). Lactococcal phages are currently classified in 12 genetically distinct group or species, but only the species 936, c2 and P335 are responsible for most large scale phage attacks worldwide (Moineau et al., J. Dairy Sci. 79:2104-2111 (1996)).
To cope with this diverse phage population, L. lactis has developed numerous natural self-defense capabilities and most of them are encoded on plasmids. These natural defense systems are currently classified in four groups based on their mode of action (Garvey et al., Int. Dairy J. 5:905-947 (1995)): blocking of phage adsorption, blocking of phage DNA penetration, restriction/modification system, and abortive infection (Abi). A resistance modification mechanism is described in U.S. Ser. No. 08/424,641. Among the four groups, Abi is believed to be the most powerful (Sing, W. S. et al., J. Dairy Sci. 73:2239-2251 (1990)). Typically, in a lactococcal abortive infection, the phage lytic cycle is terminated intracellularly by the Abi protein and the host is killed. This suicidal outcome limits phage dissemination which can be visualized by the absence of plaque or by a reduction in plaque size. So far, eleven lactococcal Abi have been characterized to the molecular level: AbiA (Hill et al., Appl. Environ. Microbiol. 56:2255-2258 (1990)), AbiB (Cluzel et al., Appl. Environ. Microbiol. 57:3547-3551 (1991)), AbiC (Durmaz et al., J. Bacteriol. 174:7463-7469 (1992)), AbiD (McLandsborough et al., Appl. Environ. Microbiol. 61:2023-2026 (1995)), AbiD1 (Anba et al., J. Bacteriol. 177:3818-3823 (1995)), AbiE (Garvey et al., Appl. Environ. Microbiol. 61:4321-4328 (1995)), AbiF (Garvey et al., Appl. Environ. Microbiol. 61:4321-4328 (1995)), AbiG (O'Connor et al., Appl. Environ. Microbiol. 63:3075-3082 (1996)), AbiH (Prevots et al., FEMS Microbiol. Lett. 142:295-299 (1996)), AbiJ (Deng et al., FEMS Microbiol. Lett. 146:149-154 (1997)) and AbiK (Emond et al., Appl. Environ. Microbiol. 63:1274-1283 (1997)), out of which only AbiD, AbiD1 and AbiF share protein homology. The availability of such a diverse group of Abi proteins most likely reflects differences in their mode of action and is probably responsive of the heterogeneity in lactococcal phage populations.
Many phage-resistant L. lactis strains have been constructed by introducing Abi systems into phage-sensitive strains. Extensive use of these strains in commercial applications led to the emergence of phages capable of overcoming these hurdles (Alatossava et al., Appl. Environ. Microbiol. 57:1346-1353 (1991); Moineau et al. Appl. Environ. Microbiol. 59:197-202 (1993)). Another Abi system is described in WO97/20917. Thus, the search for novel and natural antiphage barriers is still a high priority for dairy starter suppliers. These new mechanisms should systematically be tested against members of the 3 common lactococcal phage species to determine their strength and to assess their true potential (Moineau et al., J. Dairy Sci. 79:2104-2111 (1996)).